1 and 2 are not mutually exclusive. You have two different concerns here.
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Get notification that a sensor device has gone offline.
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Avoid using stale data in your calculations.
I still suggest that you use Expire on all these Items and have them go to UNDEF when they do not receive an update after a reasonable amount of time (usually 2 x reporting period). This addresses 2.
Does the Thing go offline when it stops reporting in all of these cases? If so, you can use that as a trigger on a Rule to send the notification.
If not something like your 2 is documented in Design Patterns: Generic Is Alive.